Monitoring and measuring apparatus for frequency modulated signals



March 30, 1948.

R. A. BRADEN MONITORING AND MEASURING APPARATUS FOR FREQUENCY MODULATED SIGNALS Original Filed Feb. 27, 1942 BY @M Hrrafwiy Patented Mar. 30, 1948 MONITORING AND MEASURING APPARATUS FOR FREQUENCY MODULATED SIGNALS Rene A. Braden, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Original application February 27, 1942, Serial No.

432,632, now Patent No. 2,369,011, dated February 6, 1945. Divided and this application August 4, 1944, Serial No. 548,083

Y 7 Claims. 1 This application is a division of applicants copending application Serial No. 432,632, filed February 27, 1942, entitled Monitoring and measuring apparatus for frequency modulated signals.

This invention relates generally to electrical measuring apparatus, and particularly to monitoring and measuring apparatus for frequency modulated signals in which observation of the peak frequency swing of such signals is of interest.

Heretofore various methods and apparatus for measuring the peak frequency swing of frequency modulated signals have been used, but all have involved the use of complicated and carefully adjusted circuits for selecting the particular frequency bands required for observation and for providing suitable indications of the instantaneous frequency values of the signals.

It is an object of this invention to provide a method and apparatus for utilizing conventional means for deriving the instantaneous frequency deviation of the frequency modulated signals with reference to a mean or reference carrier frequency source, combined with a new and simple method and apparatus for indicating the value or degree of this instantaneous frequency deviation. It is another object of the invention to provide a method and apparatus for amplitude modulating the frequency modulated signal with a reference carrier frequency to derive a beat frequency corresponding to the mean deviation between the frequency modulated signal and the reference frequency, in which the beat frequency is applied to a measuring instrument as a function of the modulating component of the frequency modulated signals. It is an additional object of the invention to provide a method and apparatus for selecting frequency limitsand indicating overmodulation, with respect to such limits, in a frequency modulated signal.

The invention will be described by reference to the drawing in which Figure 1 is a schematic circuit diagram of one embodiment of the invention, Figure 2 is a schematic circuit diagram of another embodiment of the invention, and Figure 3 is a schematic block circuit diagram of still another embodiment of the invention.

Referring to Figure 1 of the drawing, a source of frequency modulated signals is connected to the input circuit I of a modulator 2. A source of reference carrier frequency, preferably the mean carrier frequency of the frequency modulated signals, is connected to the input circuit 3 of the modulator 2. The output of modulator 2 is connected through the amplifier 4 to the control electrode of a pentode type electron discharge tube 5 having high anode resistance. A relatively high inductance 6 is included in the anode circuit of the tube 5. The voltage developed across the anode circuit of tube 5 is applied to the vertical deflecting element 'I of a cathode ray oscillograph tube I3. Since the anode load of tube 5 is a substantially pure inductance the voltage developed across the anode circuit will be directly proportional to the frequency applied to the control electrode, Therefore, the vertical deflection of the cathode ray `of the oscilloscope 3 will be directly proportional to frequency. A source of signals corresponding to the modulating component of the frequency modulated signal under observation is connected to the terminals 8 of a variable phase shifter 9. 'Ihe output of the phase shifting circuit is connected to the horizontal defleeting elements I0 of the cathode ray oscillograph |3. This horizontal deflecting signal provides a timing voltage which permits the instantaneous frequency deviation of the frequency modulated signal to be observed. The signals corresponding to the modulating component of the frequency modulated signal may be derived either directly from the frequency modulated signal input circuit or they may be derived from the transmitted signal under observation by means of a conventional frequency modulation receiver.

Referring to Figure 2 of the drawing, a frequency modulated signal to be observed is applied to the terminals 20 of a modulator 2|. A reference carrier frequency, preferably corresponding to the mean carrier frequency of the frequency modulated signals is applied to the terminals 30 of the modulator circuit 2|. The output of the modulator circuit 2| is connected to the input of a first variable gain amplifier 23 and is also connected to the input of a second variable gain amplifier 2'| The output of the first amplifier 23 is connected to the control electrode of a pentode thermionc tube 24 having high anode resistance. A substantially pure inductive load 25 is connected in the anode circuit of tube 24. The output of the second amplifier 21 is connected to the control electrode of a pentode thermionic tube 28 having high anode resistance. A substantially pure inductive load 29 is connected in the anode circuit of the tube 28. Switching means 22 are provided for connecting a peak reading'voltmeter M across the anode circuits of either of the tubes 24 or 28, as desired. Grid bias for the variable gain amplifiers 23 and 21 is obtained from a square wave limiter-amplier 33 which is actuated through a balanced cir- -r cuit 3l by signals corresponding to the modulating component of the frequency modulated signals under observation. This modulating-:com-

ponent can berobtained directly from the modu-f lation input circuit of the frequency modulated signal source. or by means of a conventional frel- 1 quency modulation receiver actuated by the frethe frequency modulated signalno phaseV shifting network is required for satisfactory timing. The amplifier 23 will conduct during periods of positive frequency deviation, and the degree of deviation will be indicated by the voltage, proportional to frequency, developedacross* the inductive load 25. as previously explained. The amplifier will become conducting on'negative frequency4 rdeviation and' the degree of deviationy wilibeindicated bythe voltage proportional tofrequencyI developedacross the inductiveload- 25.

Figure 3 of the drawing provides a method andcircuit for indicating over-modulation. A source of' frequency modulated signals under observation is connected to the terminals 481 which are connected tothev input circuits of modulators` 41 and; 3; Y Il? isl connected to the' input of therrloolulator` lil; and aY second variable4 frequencyv oscillator 411i is' connected tothe input of themodulator 431. The; frequency of the Yoscillator i121 is adjustedslightly above the-desired maximum frequency` limit of the frequency modulatedA signal. The

nals, a source of oscillations having a frequency Y exceeding a predetermined frequency Vmodulation limit of said signals, means for mixing said signalsand= sacl-` oscillations to derive/beat fre-Y quency currents, a 'low-pass l'ter network-re- Y Y sponsive to saidf mixed signals for passing only the beat frequency currents derived rfrom overmodulation of said signals exceeding said frequency limits, andian indicator responsive to signals passed' by said network. Y

3. Apparatus for-indicating over-modulation of frequencymodulated signals including incombination, connection means for a. source of frequency modulated signals, asource of oscillations` having a, frequency exceeding a predetermined.frequencymodulation limit of said signals;. means; fonmixing saidsignals and Vsaid A variable frequency oscillator frequency of thev oscillator i4 is adjusted slightly` below the minimum desired frequency limit'of thel frequency modulated signal. The output of the modulator elY is-connectedthrough aV conventional low pass filter-.(15 to an indicator 41. The output of the-'modulator i3A is connected`r through a low-passffilter MitoY an indica-tor 4B. It-'will be apparentV that if the frequency modulatedv signal under observation swings toor through'ivalues 'which produce beat frequency: currentsrin the output ofthe modulators whichv will be passed by the low-pass lters, either oney orv the other of'the indicators will beactuated. Itv will be apparent that the frequency of oscillators `lli! and M1 can be adjusted toY indicate overmodulation in either symmetrical or unsymmetricalgfreouency modulated signals; It will be understoody that the'circuitY components illustrated' in the drawing byv block diagram. and not (described, in detail, 'are of con-V ventional types Well known to` those skilled inv the art. I

L claim asv my invention: l: Apparatus for indicating over-modulation offfrequency modulated signals including in com-Y bination,Y a source of frequency modulated signais; a source of oscillations having a frequency exceeding a predetermined frequency modulation limit ofsaid'signals,` means for mixing'said sig;- n'aljs andsaid oscillations to derivebeat'frequency currents and an indicator responsive only to beatfrequency currents derived from over-mod'- ui'ati'on of saidvsignals exceeding said frequency limits.,Y

2.. Apparatus for indicating over-modulation of. frequency modulated signals -including in com,- bination, a source of frequency modulated sigoscillations to derive beat frequency currents,

a low-pass filter networkyresponsive to said mixed signals for passingzonlysthe beat frequency currents'derived fromxover-modulation of said' signais exceeding said frequency limits; and anV indicator responsiveto signalspassed by saidjnet- Work.

of'frequency modulated signals includingiin.l coin- Vbination, ai source' of frequency modulated; signals, a. source of oscillations having afrequency exceeding a predetermined, frequency modulation limit of 'said signals, means for mixing said j signals and said oscillations to derive beat frequency currents' and: an indicator responsive toV the magnitudev of only: thoseV beat frequency currents. derived from over-modulation of' said' sig-Y nals exceeding said: frequency limits.

5. ApparatusA for indicating over-modulation.

of frequency modulated signals including, in com'- binati'on, afsource. of frequency modulatedv signals, a first rsource of' oscillations having; a frequency higher thanL a' predetermined high fre-V quency modulation liinit of said signals, a second; source of oscillations-having a frequency lower than a predetermined low frequency mod-z ula-tion limit of*sai'd signals, means for mix-v ing said: signals and said-'first oscillations to'd'- rive aA first beat frequency, meansV forrmixing said signals and saidY second oscillations toA derive a second' beat frequency,v and an indicator Y ofv frequency modulated`- signals including, yin

combination, 'a source ofy frequency modulated signals, a first source of oscillations having a frequency higher than a predetermined high frequency modulation limit of said. signalsa secondsource of, oscillations having a frequency lower' than a predetermined low frequency mod-` ulation limit of said fsignals, means for mixing said signals and said rst oscillations to derive a, rst beat frequency, means for mixing said' signals and said second oscillations to de-r rive a second beat frequency, separate low passY filters for passing only beat frequency currents derived from over-modulation of said signals exceeding either of said limits, and an indicator for;

each of said beat frequency currents derived from said' filters, said indicators cooperating to indicate over-modulation above and below the mean signal frequency. p Y s 7, Apparatus for indicating over-modulation of 4. Apparatus for indicating over-modulationV frequency modulated signals including, in combination, connection means for a source of frequency modulated signals, a rst source of oscillations having a frequency higher than a predetermined high frequency modulation limit of said signals, a second source of oscillations having a frequency lower than a predetermined low frequency modulation limit of said signals, means for mixing said signals and said rst oscillations to derive a rst beat frequency, means for mixing said signals and said second oscillations to derive a second beat frequency, separate low pass lters for passing only beat frequency currents derived from over-modulation of said signals exceeding either of said limits, and an indicator for each of said beat frequency currents derived from said lters, said indicators cooperating to indicate over-modulation above and below the mean signal frequency.

RENE A. BRADEN'.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

